FIG. 2 illustrates the configurations of wireless communication apparatuses and wireless communication system to which the present invention applies. A wireless communication system comprises a wireless communication apparatus at a transmitting end 201 and a wireless communication apparatus at a receiving end 202 and provides phone calls and data transmissions via a radio propagation path 203. Although FIG. 2 and subsequent figures depict only one of two links, such as a downlink, the present invention is equally applicable to both an uplink and downlink.
In the communication apparatus at a transmitting end 201, the data to be transmitted is first encoded by a encoder 207 in a transmitter 205. For wireless communication, convolutional coding and turbo coding are frequently used. Encoded information is entered into a QAM (Quadrature Amplitude Modulation) modulator 206 and modulated. A baseband signal derived from modulation is converted to a radio-frequency band signal by a radio unit 208 and then transmitted. In the wireless communication apparatus at a receiving end 202, on the other hand, a radio signal received from the radio propagation path 203 via antenna 210 is converted to a baseband signal by a radio unit 215. The baseband signal is first demodulated by a QAM demodulator 213 in a receiver 211 and then decoded by a decoder 214. In the QAM demodulator 213, a demodulation process is performed in accordance with the channel estimation result produced by a channel estimator 212.
As regards encoding for the wireless communication apparatus at a transmitting end, it is assumed, as shown in FIG. 3, that a turbo coding method is employed at a code rate (R) of ⅓ and at a constraint length (K) of 4 (dotted lines apply for trellis termination only). Turbo coding is defined as a third-generation mobile communication coding method by 3GPP (3rd Generation Partnership Project) Specification TS 25.212 and one of the widely used coding methods.
In a QAM modulator of the wireless communication apparatus at a transmitting end, long-studied multilevel modulation (QAM modulation) is conducted in order to increase the transmission efficiency. In the case of 64-QAM, 6 bits can be transmitted per symbol. As shown in FIG. 4 (401), 6 bits (S5, S4, S3, S2, S1, and S0) are grouped into a 3-bit (S2, S1, and S0) in-phase component (I component) and a 3-bit (S5, S4, and S3) quadrature component (Q component). Gray coding is conducted so that the difference between adjacent symbols is 1 bit. Transmission takes place with the signal constellation shown in FIG. 4 employed.
In a radio propagation path, the QAM-modulated outgoing signal is affected by a combination of amplitude variation and phase rotation, which is unique to a radio propagation and called “fading”. To achieve proper demodulation and decoding in the wireless communication apparatus at a receiving end, it is therefore necessary to estimate such a variation correctly as indicated in FIG. 5. It is assumed that the QAM signal whose signal constellation is as shown in 501 is transmitted and subjected to fading having an amplitude variation of G and a phase rotation of θ as shown in 502. The resulting signal constellation is as shown in 503. At the receiving end, the pilot signal or other signal transmitted for channel estimation is used to estimate the amplitude variation G and phase rotation θ in the channel. Further, the complex conjugates of the obtained channel estimation result are multiplied together to implement −θ rotation and provide phase rotation correction as shown in 505. For a QAM or other signal on which information is superposed in the direction of the amplitude as well, it is also necessary to properly estimate the value of amplitude variation G for demodulation purposes.
To achieve channel estimation described above, the pilot signal for channel estimation is generally prepared separately from a data signal, which carries information. The pilot signal is characterized by the fact that the signal receiving end knows what type of signal is transmitted. Therefore, the received pilot signal itself indicates the channel situation. If, for instance, the pilot signal has a signal constellation of (1,0) or is expressed as 1×exp(j0), the received signal is G×exp(jθ). It is obvious that the received signal indicates the channel condition. When a signal whose quadrature component only is subjected to positive/negative inversion is generated from the received signal, the resulting signal is G×exp(−jθ). It is therefore easy to obtain a signal that is required for invoking (−θ) rotation, which is necessary for demodulation.
There are two pilot signal transmission methods. One method provides parallel transmission as shown in FIG. 6 and simultaneously transmits a data signal and pilot signal on separate communication channels. The other method, which is shown in FIG. 8, uses the same communication channel for a data signal and pilot signal while periodically inserting a pilot signal.
Channel estimation will now be described in detail while explaining how the use of a pilot signal value is timed.
When a first pilot signal is transmitted in parallel with a data signal as shown in FIG. 6, the number of data received within the same period of time varies because the data signal transmission rate is higher than that of the pilot signal. In the example shown in FIG. 6, one square corresponds to one signal symbol. While one symbol of the pilot signal P3 (601) is received, four symbols (d1, d2, d3, and d4) of a data signal (602) are received. Therefore, the simultaneously received pilot signal P3 is used as the channel estimation result for demodulating the four symbols (d1, d2, d3, and d4). Thus, the received P3 is used as the channel estimation result.
If the channel is affected by fading, noise, or the like at the time of P3 when the channel estimation method shown in FIG. 6 is used, the communication quality may deteriorate due to improper channel estimation. To avoid this problem, a method shown in FIG. 7 is used. When this method is used, a plurality of pilot signals prevailing before and after an estimation time are weighted as shown in 701 and added together (702) for use as the channel estimation result. In this case, however, simple weighting coefficients, such as W1:W2:W3:W4:W5=1:2:3:2:1, are used so as to merely give greater weighting coefficients to signals closer to a data signal (703).
If, on the other hand, pilot signals are periodically inserted using the same channel as shown in FIG. 8, the pilot signals 801, 803 exist before and after a data signal 802. Therefore, the intended purpose can easily be achieved by calculating the average value of the pilot signals 801, 802 and applying the average value, as the channel estimation result, to the entire data signal 802 between the pilot signals 801, 803.
If the pilot signals are affected by fading, noise, or the like during the use of the channel estimation method shown in FIG. 8, the communication quality may deteriorate due to improper channel estimation. To avoid this problem, a method shown in FIG. 9 can be used. This method weights a plurality of pilot signals prevailing before and after an estimation time as shown in 901 and adds them together (902) for use as the channel estimation result. In this case, however, simple weighting coefficients, such as W1:W2:W3:W4=1:2:2:1, are used so as to merely give greater weighting coefficients to signals closer to a data signal (903). An example of “W1:W2:W3:W4=1:2:2:1” use is introduced in “Channel Estimation Scheme using the Plural Pilot Blocks for DS-CDMA Mobile Radio” (Ando, et al; Institute of Electronics, Information and Communication Engineers Report RCS96-72).
The feature of the channel estimation methods described above is that the channel estimation result derived from the information about one or more pilot signals is applied to the demodulation of a plurality of data signals (symbols). When a conventional channel estimation method is used with a QAM or other modulation method by which the amplitude and phase carry information, highly accurate channel estimation results are required for demodulation as well. Therefore, if the same channel estimation result is used for a plurality of data signals (symbols), the communication quality significantly deteriorates due to incorrect channel variation estimation.
Further, even if a conventional method is used to achieve channel estimation by weighting pilot signals prevailing at a plurality of times, there are no quantitative calculation grounds for weighting coefficients. Therefore, if the coefficients are improperly set, the communication quality may seriously deteriorate.
As the speed of information transmission in a wireless communication system increases, an increasing number of systems employ the QAM method as a modulation method for the purpose of enhancing the frequency use efficiency. Under these circumstances, it is therefore an object of the present invention to provide a channel estimation method for estimating a channel efficiently and accurately and establishing communication at high quality, that is, providing communication with excellent error rate characteristics.